Microsoft Security Essentials is a new free security tool from Microsoft, released in October 2009. It's meant to provide protection not just from viruses, but also spyware, rootkits, and trojans as well.
What is Microsoft Security Essentials
You are correct, Windows Defender has been superseded by another free Microsoft utility. On September 1, 2009, Microsoft released Microsoft Security Essentials (MSE). It protects against a broader spectrum of malware than Defender does, including viruses, rootkits, and trojans as well as spyware.
Windows Defender detects and removes spyware only. It won't protect your computer from other malicious software, such as viruses, worms, and Trojans. The new Microsoft Security Essentials, on the other hand, is a comprehensive anti-malware solution. MSE is meant to replace both your existing anti-spyware and anti-virus programs. If you're running Windows Defender, you don't actually have to uninstall it. MSE is smart enough to detect and disable Windows Defender. But, if you are currently using some other anti-spyware program such as Malware Bytes Anti-Malware, SpyBot S&D or Ad-Aware, OR an anti-virus program such as Norton, McAfee, AVG or Avira, then you SHOULD disable or uninstall them before installing MSE. Anti-malware programs tend to fight with each other, if you have more than one active.
